1. Field of the Invention
This invention relates generally to a method for monitoring the fluorescence of 4-carboxy-p-terphenyl in crude terephthalic acid formed by the liquid-phase oxidation of p-xylene in a solvent, and more particularly concerns the measurement of the fluorescence of 4-carboxy-p-terphenyl separated from the crude terephthalic acid by isocratic, high performance liquid chromatography.
2. Discussion of the Prior Art
Polymer grade or "purified" terephthalic acid is the starting material for polyethylene terephthalate, which is the principal polymer for polyester fibers, polyester films, and resins for bottles and like containers. Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, e.g., palladium on a carbon support, as described in U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts the various color bodies present in the relatively impure terephthalic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in U.S. Pat. No. 4,405,809 to Stech et al.
Even after the purification of crude terephthalic acid produced by the liquid-phase oxidation of p-xylene in a solvent by the aforesaid well-known procedures, the resulting purified terephthalic acid contains impurities which fluoresce at wavelengths of greater than 370 nanometers, preferably 390-400, nanometers produced by excitation wavelengths of 260-320 nanometers. Since the concentration of such impurities in the purified terephthalic acid can vary significantly, specifications are often established for the amount of such fluorescence which can be permitted for the purified terephthalic acid product.
Known methods for measuring the aforesaid fluorescence of purified terephthalic acid are available. However, several hours are required to purify the crude terephthalic acid and obtain a sample of the resulting purified terephthalic acid that is suitable for use in fluorescence measurements. Thus, while such purification operation is being performed and samples of purified terephthalic acid are being obtained, substantial quantities of a purified terephthalic acid product which is unacceptable to customers may be made. The method of the present invention permits this problem to be avoided by measuring the fluorescence of 4-carboxy-p-terphenyl, at wavelengths of greater than 370 nanometers produced by excitation wavelengths of 260-320 nanometers, in the crude terephthalic acid before it is purified and thereby estimating and monitoring the fluorescence, at wavelengths of greater than 370 nanometers produced by excitation wavelengths of 260-320 nanometers, of the purified terephthalic acid to be produced from it.